The present invention relates to a laser drive device.
With the recent trend toward larger-capacity and higher-speed optical disk devices, demands for laser drive devices with high speed and low power consumption have increased for data recording/reproduction for such optical disk devices.
As an example of conventional laser drive devices intending to increase the switching speed, Japanese Patent Publication No. 7-95610 discloses a laser drive device as shown in FIG. 9.
In the conventional laser drive device, an inflow-current I5A flowing into an inflow-current source 5 and a set current I5B flowing from an outflow-current source 6 are set based on a set current I4 flowing from a current setting circuit 4. An outflow-current I6 flowing from the outflow-current source 6 is set based on the set current I5B. Recording signals reverse to each other are applied to bases of transistors 3A and 3B of a differential current switch 3. When the transistor 3A is turned ON and the transistor 3B is turned OFF, a current value of a current 13A flowing through the transistor 3A becomes equal to that of the inflow-current I5A and the outflow-current I6. As a result, a value of a laser current I1 becomes zero, thereby turning OFF a laser 1. When the transistor 3A is turned OFF and the transistor 3B is turned ON, a current value of a current I3B flowing through the transistor 3B is made equal to that of the inflow-current I5A, while a current value of the current I3A flowing through the transistor 3A becomes zero. As a result, a current value of the laser current I1 becomes equal to that of the outflow-current I6, thereby turning ON the laser 1.
The above mentioned conventional laser drive device satisfies desirable conditions for driving a laser, where the laser 1 is grounded on one side and is connected to the transistor 3A as a switching element in a collector follower manner on the other side. Moreover, while satisfying the above conditions, the transistor 3A as the switching element is made of an NPN transistor having a high switching speed. This enables easy attainment of a switching speed as high as several nanoseconds or less.
The conventional laser drive device shown in FIG. 9 however has the following problem.
In general, in a laser drive device of data recording/reproduction for an optical disk device, the values of a laser current vary among the operations of reading, erasing, and writing. The current value is large during writing, while it is small during reading.
In the illustrated conventional laser drive device, the laser current I1 itself is turned ON/OFF by the differential current switch 3. Therefore, as the laser current I1 is greater, power consumption of the laser drive device increases.
As the laser current I1 is smaller, the currents flowing into the transistors 3A and 3B of the differential current switch 3 decrease, resulting in reducing the switching speed of the transistors 3A and 3B.
An object of the present invention is providing a laser drive device capable of suppressing increases in power consumption even on an increase in laser current.
Another object of the present invention provides a laser drive device suppressing decline in switching speed even on decrease in laser current.
The laser drive device of the present invention includes a laser, a first current source, a second current source, a current amplifier, a first transistor, and a second transistor. The first current source supplies a first current having a current value associated with a set current value. The second current source receives a second current having a current value associated with the set current value. The current amplifier amplifies a current from the first current source to generate a laser current and supplies the laser current to the laser. The first transistor is connected between the first current source and the second current source. The second transistor is connected between a power supply node receiving a power supply voltage and the second current source. The first and second transistors are turned ON/OFF complementarily.
In the above laser drive device, when the first transistor is OFF, a first current from the first current source is supplied to the current amplifier. The current amplifier amplifies the current supplied from the first current source to generate a laser current. The laser current is then supplied to the laser. Thus, the laser is turned ON. During this time, the second transistor is ON, allowing a second current to flow from a power supply node into the second current source through the second transistor. When the first transistor is ON, the entire or part of the first current flows into the second current source through the first transistor. This reduces the current supplied to the current amplifier and thus reduces the laser current, resulting in turning OFF the laser. During this time, the second transistor is OFF. The values of the first and second currents are determined by a set current value. The value of the laser current supplied to the laser during the ON-state of it is determined by the value of the first current. Therefore, by adjusting the set current value, a desired value of laser current can be supplied to the laser.
Since the laser drive device is provided with the current amplifier, the values of the first and second currents are smaller than the value of the laser current. This suppresses an increase in power consumed by the first and second power sources and the first and second transistors.
Preferably, the anode of the laser is connected to the power supply node, and the current amplifier includes first, second, and third NPN transistors. The first NPN transistor is connected between the first current source and a grounding node receiving a grounding potential with the emitter being grounded. The second NPN transistor has a collector connected to the power supply node, an emitter connected to the base of the first NPN transistor, and a base connected to the collector of the first NPN transistor. The third NPN transistor is connected between the cathode of the laser and the grounding node with the emitter being grounded. The base of it is connected to the base of the first NPN transistor.
Preferably, the current amplifier further includes a plurality of fourth NPN transistors connected between the cathode of the laser and the grounding node in parallel with the third NPN transistor with the emitters being grounded. The bases of the fourth NPN transistors are connected to the base of the first NPN transistor.
In the above laser drive device, the first, second, and third NPN transistors make a current mirror circuit. The current from the first current source flows through the first NPN transistor. A current of a value obtained by multiplying the current flowing through the first NPN transistor by the mirror ratio flows through the third NPN transistor, to be supplied to the laser as the laser current.
Further, the first and second NPN transistors and each of the plurality of fourth NPN transistors make a current mirror circuit. The sum of the currents flowing through the respective fourth NPN transistors and the current flowing through the third NPN transistor is supplied to the laser as the laser current.
Preferably, the anode of the laser is connected to the power supply node, and the current amplifier includes first, second, and third n-channel MOS transistors. The first n-channel MOS transistor is connected between the first current source and a grounding node receiving grounding potential. The second n-channel MOS transistor is connected between the power supply node and the gate of the first n-channel MOS transistor. The gate of the second n-channel MOS transistor is connected to the first current source. The third n-channel MOS transistor is connected between the cathode of the laser and the grounding node. The gate of the third n-channel MOS transistor is connected to the gate of the first n-channel MOS transistor.
Preferably, the current amplifier further includes a plurality of fourth n-channel MOS transistors connected between the cathode of the laser and the grounding node in parallel with the third n-channel MOS transistor. The gates of the fourth n-channel MOS transistors are connected to the gate of the first n-channel MOS transistor.
In the above laser drive device, the first, second, and third n-channel MOS transistors make a current mirror circuit. The current from the first current source flows through the first NPN transistor. A current of a value obtained by multiplying the current flowing through the first NPN transistor by the mirror ratio flows through the third NPN transistor, to be supplied to the laser as the laser current.
Further, the first and second n-channel MOS transistors and each of the plurality of fourth n-channel MOS transistors make a current mirror circuit. The sum of the currents flowing through the respective fourth n-channel MOS transistors and the current flowing through the third n-channel MOS transistor is supplied to the laser as the laser current.
Preferably, the above laser drive device further includes a third current source connected to a node interconnecting the first transistor and the second current source for receiving a third current.
In the above laser drive device, when the first transistor is OFF, the second transistor is ON, allowing the sum of the second and third currents to flow through the second transistor. The second current flows into the second current source while the third current flows into the third current source. When the first transistor is ON, the second transistor is OFF, allowing the entire or part of the first current from the first current source to flow through the first transistor. This current is equal to the sum of the second and third currents, where the second current flows into the second current source while the third current flows into the third current source.
Overall, as the value of a current flowing through a transistor is smaller, the switching speed of the transistor is lower. In the above laser drive device, as the value of the laser current supplied to the laser is smaller, the value of the second current decreases. However, since the above laser drive device is provided with the third current source, the constant third current flows into one of the first and second transistors even when the value of the laser current is small. Thus, the switching speed of the first and second transistors is suppressed from decreasing even when the laser current is small.
Preferably, the above laser drive device further includes a first diode and a voltage application means. The first diode has an anode connected to the first current source and a cathode connected to the current amplifier. The voltage application means applies a predetermined voltage in the forward direction with respect to the first diode.
In the above laser drive device, with the placement of the first diode, reverse current flow from the current amplifier is prevented.
However, an intense reverse bias may undesirably be applied to the first diode. To avoid this occurrence in the laser drive device, it provides the voltage application means to apply a voltage on the first diode that is too low to turn ON in the forward direction with respect to the first diode.
The above form provides the following additional effect.
When the laser is OFF, that is, the first transistor is ON, the voltage at the node interconnecting the first transistor and the first current source decreases. This decrease is however only to the level of a voltage applied by the voltage application means. Therefore, when the first transistor is turned OFF next, the time required for the voltage at the interconnecting node to reach a predetermined level is shortened, compared with the case of having no voltage application means. That is, the switching speed can be made higher.
It is preferable that the voltage application means includes a fourth current source, m pieces of second diode, and a third transistor. The fourth current source supplies a fourth current. The m pieces of second diode are connected in series between the fourth current source and the cathode of the first diode. The third transistor has a collector connected to the power supply node, an emitter connected to the anode of the first diode, and a base connected to the fourth current source.
In the above laser drive device, a difference voltage between a dropped voltage at the m pieces of second diode due to the fourth current and a base-emitter voltage at the third transistor is applied in the forward direction to the first diode.
Preferably, the voltage application means further includes n pieces of third diode connected in series between the emitter of the third transistor and the anode of the first diode.
In the above laser drive device, the voltage that is applied in the forward direction to the first diode is lower by a dropped voltage at the n pieces of third diode than the difference voltage between a dropped voltage at the m pieces of second diode due to the fourth current and a base-emitter voltage at the third transistor. Therefore, the first diode can be turned OFF without fail even in the case where the first diode fails to be sufficiently turned OFF with the voltage applied thereto using only the m second diodes and the third transistor.
Preferably, the laser drive device further includes a fifth current source connected to the cathode of the first diode for receiving a fifth current.
If the fifth current source is not provided, the fourth current may be supplied to the current amplifier when the laser is OFF, that is, the first transistor is ON, possibly resulting in turning ON the laser.
In the above laser drive device, the entire fourth current or part of it flows into the fifth current source as the fifth current. Therefore, the above mentioned inconvenience can be avoided.
Preferably, the voltage application means further includes a resistor connected between the emitter of the third transistor and the anode of the first diode.
In the above laser drive device, a voltage is applied to the forward direction of the first diode that is lower by a value of a dropped voltage at the resistor than the difference voltage between a dropped voltage at the m pieces of second diode due to the fourth current and a base-emitter voltage at the third transistor. Therefore, the first diode can be turned OFF -without fail even in the case where the first diode fails to be sufficiently turned OFF with the voltage applied thereto using only the m second diodes and the third transistor.